The parietal eye of lizards extracts, processes and sends to the brain information about the light striking its retina. The retina lacks bipolar cells and has only one plexiform layer. Photoreceptors synapse directly onto ganglion cells. Afferent signal flow is modified by the activity of efferent fibers. It is proposed to identify the morphological and physiological substrates for visual information processing in this retina. The morphological methods to be employed include: 1) three-dimensional reconstruction of serial electron micrographs to identify synaptic connectivity, 2) immunocytochemistry to localize possible transmitter substances, and 3) horseradish peroxidase histochemistry to differentiate neural connectivity. Both extracellular and intracellular recording techniques will be utilized to examine: 1) the physiological characteristics of photoreceptors and neurons and, 2) the information processing of synaptic connections. This proposal focuses on the parietal eye as a sensory and organ involved in visual information processing. The relatively simple neural organization of the parietal eye contrasts with the extremely complex organization of the retina. I expect that a close analysis of the parietal eye will reveal similarities and differences which viewed in comparative light may reveal important and generalizable features of sensosy processing. The health-related aspects of this work are indirect, deriving mainly from an increased understanding of basic mechanisms of retinal function.